this makes sure that any diff implementation obeys the semantics we expect
at higher layers
Change-Id: Iae8842cfb9fece94ea71c04ec146d825eff0cbeb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/191017
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>